1. Field of the Invention
The present invention relates to a printing apparatus and method that forms images by forming a number of dots on a printing medium and more particularly to a printing apparatus and method which can form high-quality images.
2. Description of the Related Art
Image forming devices widely used in printers, copying machines, facsimiles and recording apparatuses produce images from dot patterns on a printing medium, such as paper, plastic films and cloth, according to image information entered. Such image forming devices may be classified, according to the printing method employed, into a wire dot printing, a heat transfer printing, a heat-sensitive printing, an ink jet printing, and an electrophotographic printing. Of these printing methods, the heat transfer printing and the ink jet printing have in recent years found a wide range of applications as the image forming devices because of their ability to realize a high resolution at low cost.
In the above image forming devices, a print head 1 having a plurality of printing elements A-H that constitute an image forming unit forms dots a-h, as shown in FIG. 1. This process is repeated during a scanning (main scanning) to cause each of the printing elements A-H to form further dots (in FIG. 1, (1) to (5)).
FIG. 2 is an external perspective view showing an example of the image forming device equipped with a mechanism that performs the printing explained above. In this figure, reference numeral 20 denotes a print head having a group of ink ejecting openings or orifices as printing elements that are disposed opposite the print surface of a printing medium fed onto a platen 24 and print on the medium. Reference numeral 16 denotes a carriage that holds the print head 20. The carriage 16 is connected to a part of a drive belt 18 that transmits a driving force of a drive motor (main scan motor) 17, and is slidably supported on two parallel guide shafts 19A, 19B so that the print head 20 can reciprocally travel over the entire width of the printing medium. The print head 20 during its reciprocal movement prints an image according to received data on the printing medium. After each main scan operation is completed, the driving force of a subscan motor 22 is transmitted through a transmission mechanism 23 and a feeding mechanism to the printing medium, which is fed a predetermined distance in a subscan direction. Denoted 26 is a recovery unit 26 to maintain the ink jet print head 20 in good condition, which has a cap 26A for capping the print head 20 while the head is not in use or for accepting ink from the head for recovering its ink discharging performance during a recovery operation by ink sucking, and also a blade 31 for wiping a head surface where ink ejecting portions open.
A construction commonly used to obtain a main scan direction position of the carriage relative to the printing medium includes a linear scale provided parallel to the guide shaft and an encoder provided to the carriage to read the linear scale. A personal recording device for which low cost is most desired does not adopt the above construction but instead includes means for detecting the reference position of the carriage and uses a pulse motor in place of the drive motor, in order to detect the amount of displacement from the reference position in an open loop by checking the number of pulses applied to the pulse motor.
In the above conventional recording devices, however, manufacturing variations of a rotary drive source such as a pulse motor and of a motor driver that controls it are known to cause cyclic variations in the driving state. These in turn cause small cyclic variations in the speed of the carriage. Hence, the dot forming positions on the printing medium, where dots are formed by the print head as it is reciprocally moved by the carriage, cyclically deviate from the correct dot forming position.
FIG. 3 shows the amount of deviation of each dot from the correct dot forming position ("0" position), when data to be printed by forming dots at equal intervals in the main scan direction is printed by an image forming device that inherently produces such deviations. Dot numbers are numbers beginning with 0 that are assigned to the dots aligned in the main scan direction.
FIG. 4 shows distances between adjacent dots that are printed in a manner described above. As shown in this diagram, dots whose intervals are large gather locally in one area while those with small intervals locally concentrate in another area, with the large-interval area and the small-interval area occurring periodically, causing variations in the dot distribution. This in turn causes tonal variations or unevenness in the printed image, which are particularly noticeable when gray scale print data is printed.